In the next few weeks, officials at Dulles International Airport will present federal officials with a soaring 25-story air traffic control tower that is nearly twice as tall as the airport's 1962 original.

After Federal Aviation Administration officials fill the 325-foot concrete-and-steel lookout with plane-tracking computers and hone operations in the coming months, controllers will start directing traffic sometime next year.

It will mark the start of a series of major changes as Dulles continues to expand on an 11,000-acre patch of land that only decades ago was populated by dairy cows.

Nearly 23 million passengers passed through Dulles last year, and airport officials said traffic so far this year is up 30 percent. Officials said they could handle 55 million passengers yearly if all their plans are eventually completed.

That will depend on cooperation from the market.

It's unclear how the disappearance of Independence Air, whose parent company filed for bankruptcy earlier this month, would affect Dulles. If Independence Air goes out of business, other airlines are expected to ramp up service. Airport officials said they are watching the situation but remain bullish.

"We've seen growth not only in Independence but across the board at Dulles. We've seen both international and domestic air service increase," said Tara Hamilton, the Metropolitan Washington Airports Authority spokeswoman, adding that discount carrier AirTran announced recently that it is adding service to Boston and Orlando. Officials are optimistic that the number of people using the airport will continue to increase.

"We don't know yet, of course," Hamilton said. "I think we're looking for the end of 2005 to be in the 27 million range."

While the economic forces work themselves out, airport officials are pushing ahead with a $3 billion construction project to retrofit Dulles and change the way passengers are channeled from the parking lots to their airplane seats.

The biggest changes will be the addition of a security floor and a rail system linking the terminals, massive earth-moving work that is giving passengers an eyeful as they pass by on their way to catch their flights.

"That's the major centerpiece. This is years of excavation and building and tunneling," Hamilton said. The effort, after its expected completion in 2009, will add breathing room for security lines and screening, she said.

"Now it's woefully crowded up at the ticket counter level," where pre-flight screening takes place, Hamilton said. Workers are digging space for a new "security mezzanine" that will be lower than the baggage-claim level and "be much more comfortable and convenient," she said. The trains will largely replace the mobile lounges that bring passengers closer to their gates.

A moving sidewalk stretching underground between the main terminal and Concourse B began operating last year and will remain an option for passengers. But the performance of the walkway has been spotty.

"We have been working on that to make improvements," Hamilton said. Escalators and moving walkways "require a lot of maintenance, and we're always working on them."

By the end of the month, workers are expected to finish building an additional lane in each direction in the road linking the economy parking lots to the airport entrance and the terminal. The FAA also has completed an environmental study needed to add a fourth runway, which would join two existing runways running north to south and one that runs east to west.

The new control tower will manage flights on the fourth runway, as well as on a planned fifth runway that is not part of the current construction package.

An expanding Dulles will attract businesses to the area, said James E. Bennett, the president and chief executive of the airports authority.

"Companies want to be around major transportation facilities," Bennett said, adding that firms in the Dulles corridor and along Route 28 cite proximity to the airport as "one of the top reasons they chose those locations."

As with most major airports, one effect of growth could be concerns among neighbors. Proposals to build large subdivisions near Dulles probably would play into that phenomenon if they are approved.

"It's probably going to be inevitable in the future that folks, as they move into the [area], will express concerns about operations of the airport. It might not be so much of a noise issue as an overflight issue," Bennett said. Even people who move into homes far from the airport sometimes object to having planes overhead, he said.

"As this metro area has grown, that development has moved out closer to the airport," Bennett said. "As that farmland is converted into residential and other development, we're going to see that overflight issue develop."

Dulles's air traffic controllers will move from the airport's original tower, left, to a new 325-foot structure in the distance at right sometime next year.